I've only tested 05-23 and 05-26 ISOs, both fail luks installs with the same error above.
I'll download the earliest of dev (05-11) and vanilla (05-20) ISOs and check them also.
The 18.0.4 stable KDE ISO does not have this issue, haven't checked any of the 18.1.0 beta ISOs yet, might download one of them also.
I have tested this with KDE-Dev, KDE-Vanilla and KDE 18.1.0-pre-testing ISOs.
All tests use erase disk auto partitioning followed by a selected swap config (ie no swap, swap no hibernation, swap with hibernation).
All encrypted installs with swap partitions failed with the error above.
All encrypted systems without a swap partition install and boot successfully.
No swap installs fine. Swap no hibernation crashes. Swap with hibernation crashes.
After each crash the root partition has been successfully created and luksFormatted, the swap partition is created the correct size, but the cryptsetup command must have crashed before mkswap.
Same crash using XFCE 18.1.0-pre-testing.
After the crash I manually ran the cryptsetup command ...
sudo cryptsetup -s 512 --batch-mode --force-password --type luks1 luksFormat /dev/sda3
Enter passphrase for /dev/sda3:
It worked, though I had to manually enter a password.
Just to make sure I did another 18.0.4 VM encrypted install with a swap partition, and it worked fine.
KDE 18.0.4 calamares version is 126.96.36.199-1.
KDE Dev 05-11 calamares-git version is 3.2.9.r5899.c7aa9ba05-1.
So the issue was probably introduced between these two.